International Symposium on Space Flight Dynamics ORBIT DETERMINATION AND NAVIGATION OF THE SOLAR TERRESTRIAL RELATIONS OBSERVATORY (STEREO)' Michael Mesarch & Mika Robertson (GSFC) Neil Ottenstein, Ann Nicholson, Mark Nicholson, & Douglas T. Ward (a.i. solutions, Inc.) Jennifer Cosgrove, Darla German, & Stephen Hendry, James Shaw (HTSI) Abstract This paper provides an overview of the required upgrades necessary for navigation of NASA's twin heliocentric science missions, Solar TErestrial RElations Observatory (STEREO) Ahead and Behind. The orbit determination of the STEREO spacecraft was provided by the NASA Goddard Space Flight Center's (GSFC) Flight Dynamics Facility (FDF) in support of the mission operations activities performed by the Johns Hopkins University Applied Physics Laboratory (APL). The changes to FDF's orbit determination software included modeling upgrades as well as modifications required to process the Deep Space Network X-band tracking data used for STEREO. Orbit results as well as comparisons to independently computed solutions are also included. The successful orbit determination support aided in maneuvering the STEREO spacecraft, launched on October 26, 2006 (00:52 Z), to target the lunar gravity assists required to place the spacecraft into their final heliocentric drift-away orbits where they are providing stereo imaging of the Sun. 1. Introduction The twin STEREO spacecraft were successfully launched on October 26,2006, at 00:52 Z on a Delta-I1 2925 launch vehicle from the Cape Canaveral Air Force Station. STEREO-A (Ahead) and STEREO-B (Behind) reached their mission orbits on December 15, 2006, and January 21, 2007, respectively. The NASA Goddard Space Flight Center (GSFC) Flight Dynamics Facility (FDF) is providing orbit determination and navigation support services for the STEREO spacecraft in a collaborative role with the mission lead center and mission design team from the Johns Hophs University Applied Physics Laboratory (APL). Reference 1 provides an overview and history to the STEREO Mission. The STEREO spacecraft are in leading (Ahead) and lagging (Behind) heliocentric drift-away orbits, moving away from the Earth at *22 degreeslyear. The mission orbits were acheved using a phasing loop transfer to a lunar gravity assist as was done for the FDF supported Wilkinson Microwave Anisotropy Probe (WMAP) mission in 2001. Further FDF experience in this orbit regime includes support for at least 36 lunar gravity assists for the Interplanetary Physics Laboratory spacecraft (better known as Wind) mission since its launch in 1994. Tracking support for these spacecraft is provided by NASA's Deep Space Network (DSN) X-band antennas, which use variable frequencies during spacecraft ranging. Upgrades in the Goddard Trajectory Determination System (GTDS) orbit determination and prediction software were necessary for support of the spacecraft. These upgrades included changes to the dynamic force modeling capabilities, relativistic modeling, and heliocentric modeling, as well as the implementation of new range and Doppler measurement models necessary for navigation support using the Deep Space Network (DSN) X-band service. Following the software changes, testing was performed using a small set of data with orbit characteristics similar to the STEREO orbit regimes. The satellite data used for testing was obtained from the Mars Reconnaissance Orbiter (MRO), Stardust, and Deep Impact missions. Furthermore, funding was obtained to perform an independent assessment of the STEREO orbit determination by the Jet Propulsion Laboratory's (JPL) Multi-Mission Navigation team. Key orbit solutions were obtained in an effort to further validate the GTDS upgrades. The solutions identified as being important included deliveries to APL at launch plus 24 hours (L+24h), prior to first perigee, and prior to fourth perigee. The L+24h orbit solution represented the first solution suitable for maneuver planning. The first and fourth perigee solutions were longer arcs (3 to 5 days) and important in case maneuvers were needed at the respective perigees. Orbit Determination Requirements The STEREO orbit determination (OD) accuracy requirements are associated with support of onboard attitude processing, reduction of scientific measurements, and also detailed maneuver planning. The required accuracies 1 This paper was supported by the National Aeronautics and Space Administration (NASA)/Goddard Space Flight Center (GSFC), Greenbelt, MD, under MOMS contract (NNG04DA01 C), Task Orders #I09 and #28 1 of 15 were different for the three major phases of operations - early orbit (first 24 hours), the phasing loops, and the heliocentric mission orbit. While the requirements during the first 24 hours were "best effort," the need for high precision in the orbit estimation was especially acute during the lunar phasing loop periods for planning and targeting the proper lunar gravity assists by the mission design team. The expected orbital accuracies based on pre- mission error analysis233and the required orbital accuracies for each mission phase are presented below in Table 1. For the phasing loops phase prior to STEREO'S gravity assist, the orbit determination requirements are 9.4 km accuracy around the fourth perigee, necessary to properly target the required mission orbit drift rate achieved via a lunar gravity assist. In the heliocentric mission orbit, the orbit determination requirements for science are 7500 km (30) predicted accuracy over 30 days. @ 1-year, 14 day solutions definitive: 114 km @ 1-year, 30 day solutions definitive: 89 krn The results presented in this paper span from the ascent trajectory phase, which includes launch, the trans-lunar phasing loops, to the lunar gravity assists, and the mission orbit phase. Also included are the comparisons to the results provided by JPL. Early operations experience and mission design decisions and implementation from the APL point of view are provided in References 2 through 5. 2. FDF Software Upgrades DSN provides X-band TRK 2-34 range and Doppler tracking data types for the STEREO spacecraft. The STEREO X-band TRK 2-34 range data is different from the TRK 2-34 range data previously processed in the FDF because for STEREO the DSN trackers employ linearly varying transmit frequencies. This section describes the modifications that were needed to the FDF software to process the STEREO range data. Tracking Data Definition Compensation of uplink frequency to cancel predicted Doppler shft at the spacecraft receiver is a standard practice, when required by the narrowness of the spacecraft receiver bandwidth or the vulnerability of the signal waveform to distortion. DSN tracking of the STEREO spacecraft is an example of the first situation, whle Tracking and Data Relay Satellite (TDRS) users, with pseudo-noise (PN) coded signals, are examples of the second situation. The DSN implementation of Doppler compensation uses a discrete set of predicted Doppler compensated transmit frequencies and their associated times of transmission. The actual transmit frequency at a given time is linearly interpolated from the frequencies nearest in time in the discrete set. It is a STEREO mission requirement that the DSN trackers implement Doppler compensation by the use of "ramped" frequencies. The STEREO mission uses X-band transponders on both spacecraft and is supported by the DSN using DSN TRK- 2-34 data. The TRK-2-34 data specification (Ref. 6) contains a number of data types. The data types of interest for STEREO are Type 7 data, which is sequential range data (Ref. 7), Type 17 data, which is total phase observable, and Type 9 data, which is ramp report data. Doppler data is derived from type 17 data. J. Lorah, Follow-On Analysis of Orbit Determination Accuracies During the Solar Terrestrial Relations Observatory (STEREO) Phasing Loop Mission Phase, FDF-109-011, Honeywell Technology Solutions Inc., March 22, 2006. L. Chung and T. Lee, Solar Terrestrial Relations Observatory (STEREO) Mission Orbit Study, FDF-109-017, Honeywell Technology Solutions Inc., September 22, 2006. The ramp report data contains the uplink frequency, f R' at the start of the ramp, the timestamp at the start of the ramp, t and the ramp rate, f , which is the slope of the linear ramp. The transmit frequency at an intermediate R' A time, t, during the ramp period can be found as: The complexity in the processing of observed ramped frequency range and range-rate (Doppler) data arises from the necessity to light-time correct the times used in the frequency calculation of the transmission frequency. Ths leads to the situation in which accurate round trip light-time (RTLT) values are needed to convert the raw range from range units to range in meters is itself a measurement of RTLT. The problem and solution are described in detail in the following sections. Description of Problem The DSN trackers, when not using a fixed transmit frequency, ramp the frequency at a constant slope between discrete, instantaneous changes in slope. A "ramp report" message is sent out with each change in slope. The challenge, since the frequency is dynamic, is that the RTLT must be known accurately to correctly convert the range measurement, which is explicitly a measurement of RTLT. If t,, is the RTLT, and the range measurement is being made at time tM,then the transmit frequency in use at the time the observed ranging waveform was transmitted is: However, tr, is the desired measurement, and is thus not actually known, and must be estimated from an a priori orbit ephemeris. Conversion to Observed Range For the ramped frequency case, the observed value of the two-way ramped sequential ranging assembly range observable, p,", is given by F, (t)dt, modulo M (RUs) and the equivalent ambiguous 2-way range measurement in kilometers, p, is given by where RU is the range count from the raw data. M is the length of the ranging code in range units (RUs). c is the speed of light. FT is the transmit frequency in Hz. is the reception time. t~mw, t is the transmission time from the ground station. TuP This integral is evaluated using the ramped frequency records following the procedure given in Section 13.3.2.2.2 in Reference 8.